Small, Cheaper Flash Memory

Motorola spinoff Freescale Semiconductor of Austin, TX, is using nanoscale materials to develop a new generation of flash memory that will be half the size of conventional flash devices and could cost much less. In addition, the technology would make it possible to affordably embed flash and logic devices on the same chip, a move that would save space and energy and improve chip speed. Freescale has already demonstrated a 24-megabit flash device using the materials, and it plans to produce commercial products by the end of 2008.

Flash memory, which is a nonvolatile form of memory (it requires no power to store information), is increasingly common in consumer devices. Today’s flash devices store information by applying an electric field to a “floating gate” – basically a chunk of polycrystalline silicon – at the center of a transistor. This gate is surrounded by an insulating material, which needs to be relatively thick so that small defects in it don’t allow the charge to leak out. As a result, a device like the flash-based iPod Nano, which packs four gigabytes of memory into its small frame, is still carrying around a lot of inactive material.

Freescale’s technology, described by Bruce White, manager of advanced CMOS at Freescale, this week at a nanotech conference in Boston, replaces the solid silicon gate with a large number of tiny silicon crystals separated by minute amounts of insulation. In this configuration, a defect in the insulation would let charge escape from only a couple of neighboring nanocrystals, leaving most of the stored charge intact.

The result: much less insulation is needed, so that the memory occupies half as much space. Or, in other words, a flash-based gadget can carry twice as many songs. Cutting down on insulation also decreases the voltage needed to store information. This makes it much easier to integrate flash memory with information processing on the same chip, which reduces costs since the number of steps needed to make the device drops by more than half. “It helps from a cost point of view. It helps from the density point of view,” White says. “So overall it’s a much better way of making the embedded nonvolatile memory technology.”

Jim Handy, an analyst at Semico Research in Phoenix, AZ, says there is already a large demand for such inexpensive embedded memory. “What Freescale promises to do is open up the possibility to put larger memories onto an economically sized chip,” he says.

The tricky thing about the Freescale process, Handy says, is distributing the nanocrystals. It’s like “steam condensation on a window…if they put too much on, the droplets will start touching each other and linking together, which is the last thing that they want – they want them to be all stand-alone. But if they put on too few, then they don’t get a useful number of them. That’s the secret sauce that they bring to the party.”

Albert Fazio, director of memory technology development at Intel in Santa Clara, CA, says his company is also exploring technologies – metal particles as well as silicon nanocrystals – that could replace the polysilicon gates in flash memory. But such innovations probably won’t be necessary, he says, until the end of the decade, by which time it will be difficult to shrink components further using current techniques. And flash memory leader Samsung shares that view, according to a company spokesperson.

By then, flash could have some stiff competition from other new nonvolatile memory technologies. Freescale, for one, later this year plans to commercialize an MRAM (magnetoresistive random access memory) chip, based on a type of memory that could eventually replace both the high-speed memory in a computer and flash memory in cell phones and digital cameras. MRAM stores information not by storing electrons but by changing the magnetic state of a material.

Meanwhile, Fazio says the most promising replacement for flash is something called phase-change memory, which uses lasers to switch a material between crystalline and amorphous states. This technology, he says, could scale down to the “5-to-10-nanometer range,” a fraction the size of today’s memory elements.

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